Environmental Engineering Reference
In-Depth Information
10.2.2
Cathodic Mechanism
ode (Aulenta et al.
2010
). The oxidizing species
also react with primary cationic species, viz.
Na
+
and K
+
, under biopotential leading to their
removal as salt. Biocarbonates will be formed
from the reaction between CO
2
(from air sparg-
ing or aerobic metabolism) and water which
further reacts with the cationic species form-
ing respective salts. These salts can also act as
buffering agents (Eqs. 10.10-10.12) decreasing
the chances of strong redox shifts (pH changes)
during constant reduction reactions as well as the
formation of oxidizing species. The possibility of
salt removal at BES cathode under in situ bio-
potential through salt splitting mechanism was
depicted in Eqs. 10.13 and 10.14.
Similar to anode, cathode is also involved in ef-
fective remediation of waste streams and pol-
lutants such as azo dyes, nitrobenzene, nitrates,
sulphates etc. Hypothetically, it can be assumed
that, these pollutants act as terminal electron ac-
ceptors at cathode to make the electrical circuit
closed in absence of oxygen. However, their
function as electron acceptor is based on the ther-
modynamic hierarchy. Unlike anode, cathode
chamber can be maintained under different mi-
croenvironments (aerobic, anaerobic and micro-
aerophilic) to increase the treatment efficiency
based on the nature of pollutant (Venkat Mohan
and Srikanth
2011
; Srikanth et al.
2012
). Gener-
ally, oxygen is considered as the TEA at cathodes
but in biocathodes, microorganisms will be used
as the catalyst for the terminal reduction reac-
tion. The biological redox tower shows a wide
range of TEA for the possible cathodic reduction
reactions. Depending on the terminal electron
acceptors adopted at cathode, they can be clas-
sified as aerobic and anaerobic biocathodes (He
and Angenent
2006
). However, the efficiency of
treatment as well as energy output vary among
the microenvironments studied.
In the case of aerobic biocathode operation,
aerobic oxidation process undergoing in the cath-
ode chamber results in higher substrate removal.
Consumption of H
+
and e
−
during the aerobic
metabolic process (along with oxygen as TEA)
will be higher and this in turn helps in additional
substrate removal efficiency. Manifestation of
gradual substrate oxidation at anode in response
to the cathodic function facilitates the mainte-
nance of cell potential for longer periods and this
also helps in increasing the treatment efficiency
(Srikanth and Venkat Mohan
2012
). Multiple
treatment processes undergoing simultaneously
in the system initiates the bioelectrochemical re-
actions that result in increased pollutant removal.
Oxygen as terminal electron acceptor encour-
ages the release of hydroxyl (OH
−
) ion at cathode
and increases the formation of oxidation species
(Fig.
10.2
). Formation of oxidation species and
radicals at cathode under biopotential increases
the possibility of other pollutant removal at cath-
+
−
+
−
H O
+−ₒ+++
c
a
c
a
H
OH
(10.10)
+
−
(10.11)
H O
+ₒ+
CO
H
HCO
2
2
3
− + + − −
+++ ++
ₒ+
HCO
c
H
OH
e
O
3
2
cHCO
H O
(10.12)
3
2
where 'c' is cationic species and 'a' is anionic
species
+
−
+
−
c
−+ + ₒ + +
a
E
[]
H O
E c
a
H
+
OH
2
(10.13)
+ + − −
++ ++
ₒ +− +
E c
H
OH
eO
2
E
[]
c
OH
H O.
2
(10.14)
Maintenance of cathodic pH is very crucial to sus-
tain the microbial activity at cathode,
in spite
of
continuous reduction reactions. The in situ bicar-
bonate buffering mechanism formed at cathode
helps to overcome this drop in cathodic pH which
is essential in continuing the reduction reaction
as well as maintaining the metabolic activities of
microbes. Physiologically favourable redox con-
ditions in the cathode chamber support the rapid
metabolic activities of aerobic consortia, thus
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